The field of the invention relates to speed control systems for motor vehicles.
Speed control systems are known which control the engine throttle by feedback control from an error signal derived by subtracting vehicle speed from a reference speed. Typically, feedback control is disengaged in response to vehicle braking and subsequently resumed upon operator actuation of a resume switch.
An example of such a speed control system is disclosed in U.S. Pat. No. 4,337,511 issued to Schneider et al. In response to a resume command, the reference speed is gradually ramped up to the previous reference or desired speed. A series of two ramps are utilized, the second ramp being more gradual than the first, such that the desired speed is gradually approached to, allegedly, reduce overshooting the desired speed.
The inventors herein have recognized numerous disadvantages with the prior approaches. For example, at termination of the resume mode, actual throttle position will be greater than required to maintain the desired speed because the vehicle was accelerating. Accordingly, overshoot of the desired speed will result despite the gradual approach or ramp of the reference speed to the desired speed. This disadvantage may become exasperated in speed control systems which utilize an integral of the speed error as a feedback signal because of the time required to integrate an accumulated error back to zero. Another disadvantage occurs at initiation of the resume mode when ramping up of the reference speed commences. Vehicle speed may initially droop as the speed control cable slack is "wound" in and then "lurch" as the speed control system responds to the speed error between vehicle speed and the ramped reference speed.